Momentum is with the Black Caps heading into the series deciding third game of the Twenty20 series against South Africa, says captain Brendon McCullum.

New Zealand chased down 169 in 19 overs in the second game of the series at East London yesterday, thanks mainly to opener Martin Guptill, who struck 101 not out, bringing up his century and hitting the winning runs with a four off the final ball of the game.

It was a much needed win after the side failed to deliver in the first game where they were dismissed for just 86 runs - a total chased down in the 13th over.

Between matches, McCullum said his side never waivered in their belief.

"We stayed strong to the team planning and stayed tight as a group. This was a great comeback and I'm really proud of the guys," McCullum said.

"This is a result that will hopefully leapfrog us into the final game and the series. This can't be a one-off game for us."

The victory yesterday was largely dependent on the innings played by Guptill, the second New Zealander to make a T20 international hundred - McCullum has two.

Guptill had been laid low with a stomach bug in the lead-up, and the team was feeling the heat following a poor performance in the opening game and heightened public scrutiny following the axing and withdrawal of captain Ross Taylor.

"It is great that the team turned around in 48 hours, and a great knock by Martin after being sick a couple of days back," McCullum said.

“Our bowlers did well to stop South Africa's batting line up, but no one expected Martin to play that innings after being sick."

Guptill launched into an emotional celebration on hitting the winning runs and later credited the advice of close friend Taylor for the way he batted.

He also adopted one of the former skipper’s trademark shots, the slog sweep, to great effect.

"It's the first time I've actually swept the ball in international cricket, never really played spin that well," Guptill said.

"We wanted to put a bit of pressure on him [debutant Proteas left-arm spinner Aaron Phangiso], it's his first game, and make him show his skills."

It was a tactic that worked as Phangiso conceded 42 runs off his four overs.